Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount attributable to S&P Global Inc. common shareholders:
Net income	$ 263	$ 414	$ 421	$ 399	$ 537	$ 892	$ 383	$ 294	$ 1,496	$ 2,106	$ 1,156
Basic weighted-average number of common shares outstanding (in shares)									256,300,000	262,800,000	271,600,000
Effect of stock options and other dilutive securities (in shares)									2,600,000	2,400,000	3,000,000
Diluted weighted-average number of common shares outstanding (in shares)									258,900,000	265,200,000	274,600,000
Net income:
Basic (in dollars per share)	$ 1.03	$ 1.62	$ 1.63	$ 1.54	$ 2.07	$ 3.39	$ 1.45	$ 1.11	$ 5.84	$ 8.02	$ 4.26
Diluted (in dollars per share)	$ 1.02	$ 1.61	$ 1.62	$ 1.53	$ 2.05	$ 3.36	$ 1.44	$ 1.10	$ 5.78	$ 7.94	$ 4.21
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares not included in the computation of diluted earnings per share for both restricted stock and stock options (in shares)									0	0	0
Restricted Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Outstanding shares not included in the computation of diluted earnings per share for both restricted stock and stock options (in shares)									600,000	700,000	900,000